JP3073754B2 - Heat resistant steel for engine valves - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to heat-resistant steel for an engine valve used in an internal combustion engine of an automobile or the like.

[Conventional technology]

Conventionally, exhaust valve steels include 21-4N steel (known as high-Mn heat-resistant steel, which has moderately high temperature strength, corrosion resistance to lead and sulfur contained in gasoline, and oxidation resistance, and has the advantage of low cost. 0.55C-0.2Si-9Mn-4Ni-21Cr-0.
4N) has been widely used. However, in recent years, with the increase in combustion temperature due to the high efficiency and high output of gasoline engines, the demand for heat-resistant steel for valves with even higher high-temperature strength than that of 21-4N steel has increased. (JP-B-61-20623, JP-A-60-779)
No. 64, JP-A-59-211557, JP-A-63-89645 and JP-A-1-79351. [Problems to be Solved by the Invention] The above-mentioned steels for the purpose of improving the high-temperature strength of 21-4N steel,
In each case, the C content is higher than that of 21-4N steel, and V, Nb,
The amount of addition of alloying elements such as Mo and W is increased. In order to improve the high-temperature strength, a state in which these additional elements are dissolved in a matrix in a matrix or a state in which the elements are precipitated and strengthened as fine carbides is desirable. Therefore, the presence of coarse carbides of these alloy elements is not preferable. Therefore, these improved steels
21-4N which is usually used to obtain the desired strength
1100-1150 higher than the solution heat treatment temperature of steel, 1050 ° C
A solution treatment at ℃ must be performed.

Such an increase in the solution treatment temperature requires an increase in heat consumption, damage to the furnace body and, in some cases, the need to improve existing equipment from the viewpoint of engine valve manufacturing, and leads to higher product costs. It has been desired to obtain a steel capable of obtaining sufficient high-temperature strength even at a solution treatment temperature of around 1050 ° C.

In addition, the above-mentioned improved steel, in order to improve the high-temperature strength,
Because it contains many elements harmful to oxidation resistance, such as V and Nb,
Oxidation resistance is lower than that of 21-4N steel.

The disadvantages of the existing improved steel of 21-4N steel are described below. The steel disclosed in Japanese Examined Patent Publication No. 61-20623 has a disadvantage that its oxidation resistance is lower than that of 21-4N because it contains a lot of V and Nb. The alloy disclosed in Japanese Patent Application Laid-Open No. Sho 60-77964 also has an insufficient oxidation resistance because of the large amounts of V and Nb added, and a solution treatment at a high temperature because of the large amount of C added. In such a case, although the strength is obtained, there is a problem that at the same solution treatment temperature as that of the 21-4N steel, the coarse primary carbide cannot be sufficiently dissolved to form a solid solution, and the strength is reduced.

The steel disclosed in JP-A-59-211557 also contains V as an essential component and has a problem in oxidation resistance. Further, both C and N are added for the purpose of forming carbonitrides, but the amount of N added is not shown in Examples, and particularly, the content of C is
In the solution treatment at a low temperature, which is as high as 0.65 to 0.72%, the primary carbides cannot be sufficiently dissolved to form a solid solution, and the high-temperature strength is insufficient.

The steels disclosed in JP-A-63-89645 and JP-A-1-79351 also have a problem that the C is high and the strength is not obtained by a solution treatment at a low temperature, and furthermore, Mo and W are used as solid solution strengthening elements. There was a drawback that the function and effect were not sufficiently exhibited.

An object of the present invention is to provide a heat treatment at a solution treatment temperature comparable to that of a conventionally used 21-4N steel, to provide excellent high-temperature strength and
An object of the present invention is to provide a heat-resistant steel for an engine valve having the same corrosion resistance and oxidation resistance as 21-4N steel.

[Means for solving the problem]

In view of the above-mentioned problems, the present inventor has tried to strengthen the heat-resistant steel for engine valves by solid solution strengthening of various elements rather than the conventional precipitation strengthening of carbide. As a result, C was added to the minimum necessary amount, and among substitutional solid solution strengthening elements as solid solution strengthening elements, the degree of deterioration of oxidation resistance was small, and W was most effective in improving creep strength.
Combines the interaction of N with the interstitial solid solution strengthening element to provide good oxidation resistance and high temperature strength at the same low solution heat treatment temperature (around 1050 ° C) as conventional 21-4N steel. It has newly discovered steel having the above characteristics.

One of the features of the heat-resistant steel of the present invention is the above-described 21-4N.
Unlike an improved steel, the point is that V, an element having an adverse effect on oxidation resistance, is not added.

That is, the first invention after the present invention is C0.0
1% or more and less than 0.20%, Si 0.05 to 1.0%, Mn 7.5 to 15.0%, N
One or two of i and Co are Ni + Co 2.0 ~ 20.0%, Cr15.0
~ 25.0%, Mo3.0% or less, W over 2.0% and up to 10.0, Nb0.0
1% or more and less than 0.50%, N 0.30% to 0.65%, B 0.02% or less,
A heat-resistant steel for an engine valve, comprising a steel having a composition of Fe and a balance of Fe containing unavoidable impurities, wherein the Co content is in the range of Co% = (Ni ± 5)%. The heat-resistant steel for an engine valve according to the first or second invention. The fourth invention has an oxidative weight loss of 0.15 mg / cm ² / hour or less when kept at 1000 ° C. for 100 hours in the atmosphere,
And the steel subjected to the aging treatment after the solution treatment at 1030 to 1070 ° C has a tensile strength at 900 ° C of 196MPa or more, and the creep rupture life under a stress load of 59MPa at 900 ° C of 25 hours or more. The heat-resistant steel for an engine valve according to any one of the third inventions.

[Action]

First, the reasons for limiting numerical values in the present invention will be described.

C is an extremely strong austenite-forming element, and is an element necessary for turning the matrix into austenite and increasing the strength, so that at least 0.01% is required. However, as the amount of C increases, the amount of carbides increases, and when it exceeds 0.20%, at a low solution treatment temperature of about 1050 ° C., the solid solution of carbides is not sufficiently performed, and the added alloy element strengthens the matrix. Useless. In order to improve the creep strength at high temperatures, it is desirable that the crystal grains grow to an appropriate size,
Such an increase in carbides suppresses grain growth and is ineffective for creep strength.

Further, excessive addition of C will reduce the solid solubility of N, which is the main strengthening element of the steel of the present invention, so that the range of C is limited to 0.01 to less than 0.20%. One of the great features of the present alloy is that the amount of carbon is limited to such a low level in the heat-resistant steel for bubbles.

Si is a deoxidizing agent at the time of dissolution and an element effective for imparting oxidation resistance at a high temperature, and requires at least 0.05%. However, Si exceeding 1.0% is harmful to the corrosion resistance of PbO contained in leaded gasoline and is not effective for high-temperature strength. Therefore, the range of Si is set to 0.05 to 1.0%.

Mn stabilizes the austenite of the base and increases the cost of N
Acts as an alternative to i and Co. Further, Mn has a remarkable effect of improving the corrosion resistance to PbO in combination with N, so that at least 7.5% is required. However, if it exceeds 15.0%, a harmful sigma phase is likely to be precipitated due to a synergistic effect with Cr, so Mn is set to 7.5 to 15.0%.

Cr is an element indispensable for improving the corrosion resistance and oxidation resistance of heat-resistant steel for valves, and requires at least 15.0%. However, if the content exceeds 25%, the sigma phase is likely to precipitate, so that Cr is 15.0 to 25.0.
%.

Ni and Co are elements necessary for stabilizing austenite of the matrix, and at least one of Ni and Co is required to be 2.0% or more in order to maintain strength, corrosion resistance, and oxidation resistance.

However, excessive addition of Ni and Co reduces one or two of Ni and Co in order to reduce the solid solubility of N, which is the main strengthening element of the steel of the present invention, and to make the steel expensive. 2.0 with Ni + Co
Limited to ~ 20.0%.

Also, in the steel of the present invention, Ni and Co can obtain good properties even when used alone, but it was found that the strength was highest when both were contained at a weight ratio of approximately 1: 1. Desirably, Ni and Co are added in substantially equal amounts within the range of Co% = (Ni ± 5)%.

Mo dissolves in the matrix as a substitutional atom, and at the same time, partially forms carbide to maintain high-temperature strength. However, as will be described below, since W works clearly more effectively, it is not always necessary to add Mo. Even when Mo is added, it is limited to 3.0% or less.

W is a homologous element of Mo and, like Mo, forms a solid solution in the matrix as a substitutional atom, and at the same time, partially forms carbide to maintain high-temperature strength. However, since W has twice the atomic weight of Mo, the diffusion rate at high temperatures is low, and as a result, the effect of improving the creep rupture strength is large. Further, W, which is a substitution type solid solution strengthening element, further contributes to improvement in high temperature strength by interaction with N which is an interstitial type solid solution strengthening element, as compared with the case of adding each alone.

For the above reasons, W is an essential additive element of the steel of the present invention, and if it is 2.0% or less, sufficient high-temperature strength cannot be obtained.
Since the addition of W exceeding 0.0% does not bring about a sufficient effect and merely unnecessarily increases the specific gravity and the price of steel, W is 2.0%.
% To not more than 10.0%.

Nb forms fine primary carbides that are stable up to high temperatures, prevents austenite from coarsening, and provides an appropriate crystal grain size. As a result, good high-temperature tensile strength and creep rupture strength are obtained. Therefore, the necessary amount of Nb is 0.01% or more, but if it is 0.5% or more, the oxidation resistance is significantly deteriorated. Therefore, the Nb content is made 0.01% or more and less than 0.5%.

N is a strong austenite forming element along with C,
In the steel of the present invention, unlike C, it hardly forms a compound with alloying elements such as Nb, Mo, W, and Cr, and works as an interstitial solid solution strengthening element. Therefore, even at the solution heat treatment temperature of around 1050 ° C., which is the purpose of the steel of the present invention, almost all the added amount is useful for solid solution strengthening of the matrix, and together with the substitutional solid solution strengthening element described above,
Works very effectively to improve high temperature strength. To provide such an effect, N must be at least 0.30% or more, but within the composition range of the steel of the present invention, the solid solubility of N is at most 0.65%.
Therefore, N is limited to 0.30 to 0.65%.

When B is added in a small amount, it segregates at the crystal grain boundaries and contributes to improving the creep rupture strength and hot workability, but the effective amount for that purpose is 0.02% or less.

The heat-resistant steel for an engine valve according to the present invention is an iron-based alloy composed of the above-mentioned main elements, the following unavoidable impurities, and the balance Fe.

P ≦ 0.04% V ≦ 0.1% Ca ≦ 0.02% S ≦ 0.03% Ta ≦ 0.1% Cu ≦ 0.30% Mg ≦ 0.02 Next, the reason for the numerical limitation of the third invention of the present invention will be described.

In the present invention, the steel having the above composition is formed into a desired shape by ingot casting, forging or rolling after melting and refining. Then, 21-4N
After a solution treatment for 15 minutes to 60 minutes in a temperature range of 1030 to 70 ° C. which is a standard solution treatment temperature of steel, the steel is rapidly cooled. And
It is heated again and aged at around 750 ° C for 1 to 4 hours before use.

The heat-resistant steel for engine valves obtained in this way is
Corrosion and oxidation resistance comparable to that of 21-4N steel and Japanese Patent Publication No. 61-20623
In order to have high-temperature strength equal to or higher than that of the steel disclosed in the above-mentioned publication, it is desirable to simultaneously satisfy the following characteristics. That is, the steel of the present invention is
Oxidation weight loss after holding for 0 hours is 0.15 mg / cm ² / hour or less, and steel subjected to aging treatment after solution treatment at 1030 to 1070 ° C. has a tensile strength at 900 ° C. of 196 MPa or more or 9%.
Creep rupture life under stress of 59MPa at 00 ℃ is 2
5 hours or more. If at least one of the above-mentioned high-temperature characteristics is not achieved, the value is limited to 0.15 mg / cm ² / hour or less, 196 MPa or more, and 25 hours or more, because it is insufficient as heat-resistant steel for engine valves.

〔Example〕

The steel of the present invention, the comparative steel and the conventional steel were melted in an air induction furnace, made into a 10 kg ingot, and then forged into a 30 mm square bar by heating at 1100 ° C. The solution treatment was carried out by air cooling after holding at 1050 ° C. for 30 minutes and further holding at 750 ° C. for 4 hours, followed by air cooling aging treatment. Then, it processed into a predetermined test piece shape, and was used for the experiment. Table 1 shows the composition of each sample, and Table 2 shows the experimental results. Samples Nos. 1 to 5 and 31 to 37 are steels of the present invention, Nos. 11, 12 and 41 are comparative steels, and Nos. 21 and 22 are conventional steels. Of the conventional steels, No. 21 is a 21-4N steel and No. 22 is
Japanese Patent Publication No. 61-20, superior in creep rupture strength to 21-4N steel
623 is a high Mn heat resistant steel.

The steels of the present invention all exhibit excellent high-temperature strength, corrosion resistance, and oxidation resistance. Nos. 1 to 5 have a C level of 0.15%, and No. 31
-37 are those where C is at the 0.05% level. Comparing No. 1-5, No. 2 containing 3% each of Ni and Co is Ni and Co respectively.
No. 1 and No. 5 containing 6% each alone, have higher proof stress and tensile strength at 900 ° C., and the effect of adding Ni and Co in combination is greater. No. 33 containing 9% Ni and 6% Co is 12% Ni
No.35 containing No.3 and 3% Co has higher proof stress and tensile strength at 900 ℃ than No.33. When comparing No.33 and No.35, the ratio of Ni to Co is higher even for the same (Ni + Co). It can be seen that the closer to 1, the higher the high temperature strength. From these examples, Co% = (Ni ±
5) It can be seen that when the percentage is satisfied, the strength is further increased.

Also, as in the case of No. 3, when the amount of N decreases, the high-temperature strength tends to decrease, which indicates that N plays an important role in the steel of the present invention. No. 31 and No. 32 also have the same amount of N as No. 3, but in these alloys, a decrease in strength due to a decrease in N is minimized by a decrease in the C amount and an increase in the W equivalent.

Comparative steel No. 12 is the Mo equivalent (Mo + 1) of Nos. 1 to 5 of the steel of the present invention.
/2W)=2.2 higher than 2.2 (2.45%). Although the tensile strength at 900 ° C. is high, the creep rupture strength at 900 ° C. is inferior to the steel of the present invention. This is the effect of W contained in the steel of the present invention, and it can be seen that good characteristics as a valve material intended by the present invention cannot be obtained with Mo alone. No.31
And No. 32 also show that the creep rupture life is slightly reduced by partially replacing W with Mo. further
It can also be seen that the creep rupture life is longer as the W amount is higher as in No.36.

Comparative steel No. 11 has the same (C +) as steel Nos. 1 to 5 of the present invention.
N) Although it is an amount, the composition is higher in C and lower in N than the steel of the present invention. This alloy No. 11 contains a large amount of coarse carbides of W, Mo, and Cr. In the solution treatment at 1050 ° C., such coarse carbides do not form a solid solution and do not sufficiently contribute to strengthening the matrix. In addition, since this coarse carbide suppresses crystal grain growth, the creep rupture life was less than half the level of the steel of the present invention. Further, since the N amount is low, the corrosion weight loss value of PbO is slightly large.

Comparative steel No. 41 is the steel of the present invention No. 2 in which the amount of Nb was increased, and a high temperature strength obtained a good value, but the oxidation resistance was significantly inferior to the steel of the present invention, It can be seen that excessive addition is harmful to the oxidation resistance.

It can be seen that the steel of the present invention is significantly improved in proof stress, tensile strength and creep rupture strength at 900 ° C. as compared with the conventional steel No. 21 (21-4N steel).

The conventional steel No. 22 has a creep rupture life at 900 ° C. of less than half that of the steel of the present invention in the solution treatment at 1050 ° C., and the original properties of the steel No. 22 cannot be sufficiently brought out by the solution treatment at low temperature. Further, it can be seen that this steel contains many elements harmful to oxidation resistance, such as V and Nb, for improving the high-temperature strength, and the oxidation resistance is greatly reduced as compared with other steels.

〔The invention's effect〕

According to the present invention, it is possible to provide a steel which can be heat-treated at a relatively low solution treatment temperature comparable to that of a conventionally used 21-4N steel and has a high temperature strength superior to that of a conventional steel. Can be. In addition, the steel of the present invention has the same characteristics as the 21-4N steel in corrosion resistance and oxidation resistance, and has both excellent corrosion resistance and oxidation resistance as compared with existing high-temperature strength improved steel. Therefore, if the steel of the present invention is used, the operating temperature of the automobile engine valve can be raised, and as a result, a high-output, high-efficiency engine can be manufactured.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] (1) In mass%, C 0.01% or more and less than 0.20%, Si 0.0
5 to 1.0%, Mn 7.5 to 15.0%, one or two of Ni and Co are N
2.0 to 20.0% for i + Co, 15.0 to 25.0% for Cr, 3.0% or less for Mo, W
2.0% or more and 10.0 or less, Nb0.01% or more and less than 0.50%, N0.30
A heat-resistant steel for an engine valve, comprising a steel having a composition of the balance of Fe containing 0.6% to 0.65%, B 0.02% or less, and unavoidable impurities. 2. The heat-resistant steel for an engine valve according to claim 1, wherein the content of Co is in the range of Co% = (Ni ± 5)%. 3. An oxidative weight loss of 0.15 mg / cm ² / hour or less when held at 1000 ° C. for 100 hours in the atmosphere;
After the solution treatment at 300-70 ° C, 9
The heat-resistant steel for engine valves according to claim 1 or 2, having a creep rupture life of 25 hours or more under a stress load of 196 MPa or more at 00 ° C and 59 MPa at 900 ° C.